1. Field of the Invention
The present invention relates to a broadband parametric amplifer; and more particularly, to a parametric amplifier having a beam lead dual Schottky barrier diode fabricated on a single semiconductor chip.
2. Description of the Prior Art
Parametric amplification refers to a class of amplifiers where amplification at microwave frequencies has been achieved, wherein the output power is derived from an A.C. input, and derives its name from the fact that the differential equation governing the circuit behavior contains one or more reactive parameters which are non-linear or time varying. Parametric amplifiers can operate on the basis of periodically varying the capacitance of a Schottkybarrier varactor diode by means of a high frequency pump signal and transferring energy to a lower frequency microwave input signal by mixing of the input frequency with the much higher pump frequency through an idler frequency. The Manley-Rowe relations show that the current must flow at the three frequencies satisfying the relation:
Pump Frequency = Signal Frequency + Idler Frequency for gain to exist at the signal frequency.
A parametric amplifer employs one or more varactor diodes and a pump frequency source to obtain gain with extremely low noise figures. The pump frequency source strongly modulates or pumps the capacitance of the varactor diode at the pump frequency rate. Sum (upper sideband) and difference (idle) frequencies are generated due to the nonlinearity of the varactor diodes. The idle frequency circuit is reactively tuned (no resistive loading) for maximum current flow at the idle frequency while the upper sideband frequency circuit is adjusted for zero current flow at that frequency to achieve maximum gain and low noise figure characteristics.
In order to achieve broadband amplification, the idler circuit is preferably simple as possible consistent with having the required wide bandwidth characteristics. A dual diode parametric amplifier satisfies this requirement since filtering of the input signal in the signal circuit is not required due to the balanced nature of this circuit and, as series resonance of the diodes is employed for the idle frequency circuit, broadband operation results. The parasitic case capacitance associated with varactors packaged in a case limits the bandwidth of parametric amplifiers. Similarly the magnitude of the parasitic lead inductance of the varactor limits the resonant frequency of the idle circuit and hence limits the low noise figure values otherwise attainable. Additional resonators may be placed in the idle frequency circuit to raise this frequency, however, the bandwidth will be reduced.
Previous attempts to obtain broadband operation with high idle frequencies are described in the technical literature. For example, the paper entitled "A Millimeter Wave Varactor With Low Parasitics" by J. A. Calviello, P. R. Liegey, and B. Smilowitz, published in the Proceedings of the IEEE, Proceedings Letters Sections, March 1971, describes separate semiconductor chips with back to back mounting; a quartz horseshoe is used with quasi-planar passivated varactors with a separate connection. Also, the paper entitled, "An X-Band Paramp with 0.85 dB Noise Figure (Uncooled) and 500 MHz bandwidth" by L. Dickens, presented at the 1972 G-MTT International Microwave Symposium describes an arrangement employing planar passivated diodes with separate fine pointed wire to each diode junction. Both of the above examples have arrangements requiring individual matching of diodes (capacitance and inductance). This leads to a costly amplifier. In addition, the mechanical embedment of the varactor diode into the circuit is very complex. U.S. Pat. No. 3,,842,360 to Dickens illustrates a parametric amplifier of a type employing a pair of Schottky barrier diodes fabricated on a single semiconductor chip whereby the body is formed with four bonding pads which is commonly known as flip-chip construction. In mounting the chip across the juncture of a slot line and a co-planar line, it is required that all four pads made equal ohmic contact simultaneously; mechanical pressure is applied to obtain this contact. Problems encountered with this "flip-chip" construction include, first, and inability to control and to resonate the idler in the immediate vicinity of the diodes since the idler energy is coupled to and propagates down the slot line toward the pump source and the slot line radiates and couples to spatially distributed modes both above and below the substrate. This "flip-chip" construction leads to gain instabilities, narrow band performance, and high noise figure. Second, undue difficulty will be experienced in obtaining equal ohmic contact simultaneously when the pads are pressed against the said junction; unequal ohmic contact of the four pads causes imbalance of fringing capacitance across the junction.
The present invention satisfies the need for a parametric amplifier providing amplification with extremely wide bandwidth and low noise figure characteristics.